Friction materials such as disc brake pads and brake linings are used for braking automobiles and the like. These friction materials perform their braking role by rubbing against an opposing material such as a disc rotor or brake drum, and therefore require a high coefficient of friction and favorable stability of that coefficient of friction.
The friction material is produced by mold processing of a composition comprising a fibrous substrate, a binder and a filler and the like. The fibrous substrate forms the basic matrix of the friction material, and is the component that imparts the friction material with its braking properties.
Conventionally, asbestos was used as the fibrous substrate in friction materials, but since the identification of asbestos as being a substance injurious to humans, various national laws have lead to the production of replacement materials.
Examples of fibrous substrates that have been proposed as substitutes for asbestos include friction materials comprising a plurality of fibers, including inorganic fibers, metal fibers and organic fibers and the like (for example, see Japanese Patent Laid-Open No. 2001-072961).
Of these potential substitute materials, ceramic fibers are known to offer excellent improvement in the strength of the friction material. These ceramic fibers are synthetic inorganic fibers having a heat resistance of not less than 1000° C., and specific examples include silica fibers, alumina fibers, alumina-silica fibers, and alumina-silica-zirconia fibers.
However, the reasons that asbestos is hazardous to humans include a very fine fiber diameter, and the fact that the asbestos fibers do not dissolve in vivo, but are rather retained inside the body. Accordingly, among the potential substitute materials, there is considerable concern that inorganic fibers with a fine fiber diameter and a large aspect ratio, and fibers that do not dissolve in vivo may have the same injurious effects on humans as asbestos. For this reason, friction materials have been proposed that use a biosoluble ceramic fiber as the friction material ceramic fiber (for example, see Japanese Patent Laid-Open No. 2003-301878).
However, when compared with friction materials that use a bioinsoluble ceramic fiber, friction materials that use a biosoluble ceramic fiber suffer from inferior derusting properties, and undergo considerable abrasion upon derusting of the opposing material.